1. Field of the Invention
The present invention relates to lens systems and to an image-pickup apparatus including the lens system.
2. Description of the Related Art
Recently the demand for high-quality image-pickup apparatuses (cameras), such as video cameras and digital still cameras including solid-state image-pickup devices has increased. Moreover, the optical systems used in such image-pickup apparatuses are required to be zoom lenses that have a wide angle of view and a large focal ratio.
Such a camera includes optical members, such as a low-pass filter and a color correction filter, between the rearmost lens and an image-pickup element. Therefore, the zoom lens system used for the image-pickup apparatus is required to have a relatively long back focus.
For a camera including an image-pickup element for color images, the zoom lens system is required to have good telecentricity on the image side.
An example of a zoom lens system that has good telecentricity is a retrofocus zoom lens system that includes a lens unit having negative refractive power and a lens unit having positive refractive power, in this order from the object side to the image side.
An example of a retrofocus zoom lens system has a two-group short zoom lens system that includes a first lens unit having negative refractive power and a second lens unit having positive refractive power and that carries out zooming by changing the distance between the two lens units (refer to Japanese Patent Laid-Open Nos. 59-33417, 7-209581, and 2001-4920).
The two-group zoom lens system carries out zooming by moving the second lens unit having positive refractive power and corrects the position of the image point in accordance with the zooming by moving the first lens unit having negative refractive power. Many typical two-group zoom lens systems have a zoom magnification (zoom ratio) of about two times.
To reduce the size of the entire lens unit while increasing the magnification, a three-group zoom lens system that includes a third lens unit having negative or positive refractive power on the image side of a two-group lens unit is known (refer to Japanese Patent No. 2556046, U.S. Pat. No. 6,124,984, and US Patent Published Application Nos. 2004/0150890 and 2005/0207024).
A four-group lens unit that includes a third lens unit having negative refractive power on the image side of a two-group lens unit and a fourth lens unit having positive refractive power on the image side of a three-group lens unit is known (refer to Japanese Patent No. 3315671 and Japanese Patent Laid-Open No. 8-152558).
Recently, the demand for small video cameras and digital cameras with a zoom lens system having a high zoom ratio has increased. To reduce the entire size of the camera, a retractable lens barrel in which, when the camera is not being used for image-capturing, the distance between the lens units are reduced so that the protrusion amount of the lens from the camera main body is reduced compared to when image-capturing is being carried out.
In general, when the number of lenses included in each lens unit constructing the zoom lens system is large, the length of the lens units on the optical axis is increased, and a desired barrel length cannot be achieved.
To reduce the length of the lens units on the optical axis and to reduce the length of the lens barrel, the number of lenses included in the lens units have to be reduced.
However, when the number of lenses is simply reduced, it becomes difficult to correct aberrations, such as spherical aberration and coma, related to the single-color image-forming performance. Moreover, it becomes difficult to correct chromatic aberration within the limited range of glass material. As a result, it is difficult to reduce the size of the apparatus and to obtain high quality images at the same time.
When the number of lenses included in the entire optical system is reduced, the refractive power of each lens included in the optical system is relatively increased. Therefore, sensitivity to manufacturing error increases, and processing accuracy of the lenses and assembly accuracy of the optical system have to be increased.
Conventionally, by providing the optical system with lenses that have aspherical surfaces, the number of lenses has been reduced while maintaining image-forming performance. When lenses having aspherical surfaces are used, aberration related to the single-color image-forming performance can be corrected. However, it is difficult to correct chromatic aberration that is mostly dependent on the selection of glass material.
A method of processing an aspherical surface is discussed in the embodiments of Japanese Patent Laid-Open Nos. 59-33417, 2001-4920, and 8-152558, U.S. Patent Published Application No. 2004/0150890 and 2005/0207024. In the method discussed in these documents, resin is stacked on the surface of the lens that is provided as a base. An aspherical surface is formed by pressing a metal mold having an aspherical surface against the resin layer.
However, according to these documents, the lens and resin both have negative refractive power, and chromatic aberration is not actively corrected by using the difference in the properties of the lens and resin.
A zoom lens system having excellent optical performance and an image-pickup apparatus including the zoom lens system according to the present invention include a small number of lenses, has a wide view of angle, and is capable of effectively correcting chromatic aberration.